Preparation of Concrete Or Masonry to Receive Gypsum Plaster

NWCB Technical Document 04/93 INTERIOR PLASTER Preparation of Concrete or Masonry to Receive 700-400 Gypsum Plaster

Specification Reference: Interior Gypsum Plaster ASTM C842, Sec. 8 The bond between concrete or masonry and plaster depends on good suction (ability to absorb water) or mechanical key (surface roughness), or both. The surface of the concrete or masonry must be free of dust and loose particles. The concrete or masonry cannot be coated with oil, grease, loose paint, bituminous compound, waterproofing, damp proofing, form release agents, or curing compounds. Such materials must be removed completely by sand blasting, grinding, chipping or washing with 10% muriatic acid solution, followed with a thorough rinse. If not removed, such materials can be covered with a self-furring metal plaster base, properly attached to the concrete or masonry. Too smooth a surface can cause inadequate suction or key, and must be roughened by sand blasting, grinding or chipping, or a self-furring metal plaster base must be applied. Bonding agents may be used to aid the bond of the plaster to clean concrete or masonry. The maximum thickness for plaster over a bonding agent shall be 3/8” on a horizontal surface, and 5/8” on a vertical surface. Bonding agents shall be used in accordance with the manufacturer’s recommendations, and meet applicable federal specifications. Concrete admixes, such as for damp proofing, may cause an inadequate bond between plaster and concrete.The solution in such a case is application of self-furring metal plaster base. The best assurances of an adequate bond can be achieved by making a few random test patch applications of the plaster base coat. Unforeseen bonding problems will be disclosed by the result of the test patches. Test patches are strongly recommended.

This technical document is to serve as a guideline and it is not intended for any specific construction projects. NWCB makes no express or implied warranty or guarantee of the techniques, construction methods or materials identified herein.

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